5-alkylsalicylaldoximes form stable complexes with iron, insoluble in water, thus effectively inhibiting the process of metal corrosion and rendering its surface hydrophobic. For example, Patent Description No. EP0178850 discloses an anticorrosive preparation, containing an aldoxime with two oxime groups in an onto position towards the hydroxyl group as an active substance. Oximes may be used for direct application to the metal surface, as well as in the form of a solution in a proper organic solvent or in the form of an aqueous emulsion. As the solvent in the preparation, alcohols, ethers, ketones and aromatic and aliphatic hydrocarbons may be used, e.g. ethanol, isopropanol, toluene, xylene, chloroform or 1,1,1-trichloroethane. Considering the commonness of corrosion, it is important that the preparations used for preventing this phenomenon are simple to obtain and inexpensive.
Known methods for preparation of aldoximes consist in a reaction of a proper aldehyde with hydroxylamine. The oldest of the known methods consists in adding an aqueous or methanol [1] [D. Stpniak-Biniakiewicz, Pol. J. Chem., 54, 1567-1571, 1980] solution of hydroxylamine hydrochloride [2] [D. Stpniak-Biniakiewicz, A. ukowski, Przem. Chem., 61, 446-448, 1982], [3] [G. V. Jeffrey, Soc. Chem. Ind. London, 3, 2837-71, 1974], [4] [PL117888], [5] [A. I. Vogel, Textbook of Practical Organic Chemistry/Preparatyka Organiczna, pp. 692-693, Warszawa, Wydawnictwo Naukowo-Techniczne, 1984] or hydroxylamine sulfate [6] [Toagosei Chemical Industry CO. LTD, UK1310808, 1973] to a concentrated solution of the aldehyde in an alcohol, e.g. methanol [2] or ethanol, predominantly at the boiling temperature of the reaction mixture [5], adjusting the pH in the range of 5-8. Alternatively, the synthesis was also carried out without any addition of organic solvent, introducing the aldehyde directly into a prepared hydroxylamine solution, at a temperature of 30-50° C., with an addition of a hydroxylamine stabiliser, e.g. Sn(OH)2 [6]. When the synthesis was completed and optionally the alcohol distilled off from the post-reaction mixture, the product was extracted with a neutral organic solvent non-miscible with water, e.g. diethyl ether [1], [5], or benzene or its derivatives [6]. The extract was washed with hydrochloric or sulfuric(VI) acid, then with water, until a neutral reaction was obtained. Next, the solvent was distilled off, and the product crystallised, if possible, from hexane, a hexane-benzene mixture [1] or chloroform-light gasoline mixture [5].
In order to produce the corresponding 5-alkylsalicylaldoximes, in general their producers must have a technique for preparation of the initial aldehydes at their disposal. In most cases, pure 5-alkylsalicylaldehydes are expensive or not readily available commercially—typically, they are not commercial products (apart from laboratory reagents).
One of the main methods for synthesis of 5-alkylsalicylaldehydes is the Reimer-Tiemann reaction, most often using sodium hydroxide, alkylphenol and chloroform as reagents and a water-methanol mixture as a solvent. According to standard procedure, a solution of the corresponding phenol in methanol is introduced into an aqueous NaOH solution or suspension, and then chloroform is added slowly under reflux, at the boiling temperature of methanol [1]. Alternatively, the corresponding phenol and the entire chloroform batch are introduced into the aqueous NaOH solution or suspension, and the reaction is carried out under pressure, at a temperature of 80-88° C. [7] [W. E. Smith, The Dow Chemical Company, U.S. Pat. No. 4,324,922, 1982]. In both variants, when the reaction is completed, the entire system is cooled to room temperature, unreacted NaOH and NaCl formed in the reaction are dissolved in water, and the post-reaction mixture is then acidified with sulfuric(VI) or hydrochloric acid, obtaining two layers: an aqueous phase and an organic phase. The separated organic phase is washed with water, and the aldehyde formed, depending on the applied phenol, is purified by fractional distillation under vacuum or crystallisation, e.g. from hexane [1]. In the second stage, the so-obtained aldehyde is subjected to a condensation reaction with hydroxylamine in order to obtain 5-alkylsalicylaldoxime, using the method described above.
The two-stage synthesis described above has the disadvantage of a relatively low yield of the first stage, or preparation of 5-alkylsalicylaldehyde—usually this does not exceed 35% in relation to the alkylphenol. The low yields and low conversion degree of alkylphenols are undoubtedly caused by an inhibition of the course of synthesis after reaching an equilibrium between the substrates and the products by the reaction system. It is easy to calculate that even if the yield of condensation of the so-produced aldehyde with hydroxylamine was close to 100%, the final yield of the oxime synthesis in the two-staged process: alkylphenol→aldehyde→oxime (being a product of yields of the individual stages) would not exceed 35%, and actually it could be even lower. Moreover, synthesis of aldehydes using this method leads to formation of many by-products, including polycondensation products [4]. Also, the necessity to isolate and purify the starting aldehyde is a significant inconvenience.
In 1994, a method for preparation of oximes by a reaction of magnesium salts of proper 5-alkylsalicylaldehydes with hydroxylamine, directly in the mixture after the synthesis of these salts, eliminating laborious isolation and purification of the aldehyde was disclosed [8] [D. Levin, Zeneca Limited, EP0584988A1, 1994]. At first, a salt of a proper p-alkylphenol with magnesium is prepared. To its mixture in toluene, paraform(aldehyde) is added portion-wise, and simultaneously, the methanol forming as a by-product in the reaction of these substrates is distilled off in the form of an azeotrope with toluene, achieving formylation of the onto position of the initial p-alkylphenol salt, and formation of a magnesium salt of the corresponding 5-alkylsalicylaldehyde, suspended or dissolved in toluene. Directly after formylation, at a temperature of 40-45° C., an aqueous hydroxylamine (or hydroxylamine sulfate(VI)) solution is introduced to the post-reaction mixture in order to obtain a oxime magnesium salt. In the end, the mixture is acidified with diluted sulfuric(VI) acid, and toluene is distilled off from the organic phase isolated and washed with water, yielding a raw product—the corresponding 5-alkylsalicylaldoxime, which is optionally purified by fractional distillation under vacuum or crystallisation. The method allows for obtaining the desired 5-alkylsalicylaldoximes from the starting alkylphenols in a “one pot” system with high yields, but the entire process is time-consuming and costly.